Generally, a thermal battery includes a plurality of unit cells. Each unit cell comprises an anode, a cathode, and an electrolyte interposed between the anode and the cathode. For the electrolyte, a salt molten at high temperatures is employed. At ambient temperature, this electrolyte is not ion-conductive, and therefore the thermal battery is in inactive state. When heat is applied to the unit cell to give high temperatures, the electrolyte will be in molten state and becomes an excellent ion-conductor, thereby bringing the thermal battery into active state and enabling a supply of electricity to the outside electric devices.
Thermal battery is a kind of reserve battery. The battery reaction is not advanced unless the electrolyte melts. Thus, even after 5 to 10 years or more of storage, the battery performance same as the performance right after its manufacture can be achieved. The electrode reaction of the thermal battery advances at high temperatures. Thus, the electrode reactions advance far more rapidly compared with other batteries using an aqueous solution electrolyte, an organic electrolyte, and the like. Therefore, thermal batteries have excellent discharge characteristics under high load. Further, thermal batteries are advantageous in that electricity becomes available in a short period of time, within a second, when an activation signal is sent to the battery at usage, though the period of time varies depending upon the heating method. Thus, based on these advantageous characteristics, thermal batteries are suitably used as a power source for various ordnance devices such as a guidance system, or as an emergency power source.
To improve the above characteristics, there has been examined various thermal batteries in which iron disulfide is used as a cathode active material. To improve the characteristics of such cathode active material, there has been proposed in Japanese Laid-Open Patent Publication No. Hei 5-242896 to use a composite material of iron disulfide and iridium disulfide (iridium content: 5 to 20 wt %) for the cathode active material, for example. In the specification of Japanese Patent No. 2847982, there has been proposed to use a composite material of iron disulfide and titanium disulfide (titanium content: 5 to 20 wt %) for the cathode active material. In the specification of Japanese Patent No. 2847983, there has been proposed to use a composite material of iron disulfide and vanadium disulfide (vanadium content: 5 to 20 wt %) for the cathode active material.
The voltage of a unit cell under a practical range of current density (approximately 0.5 to 2 A/cm2) for a general thermal battery in which iron disulfide is used for the cathode active material and lithium metal is used for the anode active material is about 1.8 to 2 V. In many cases, thermal batteries are used as a power source for devices which necessitate a high output and a high load, i.e., several tens to hundreds of volts. Therefore, a stack has to be formed by stacking a plurality of unit cells and connecting the stacked cells electrically in series, to obtain the required voltage.
Recently, with the devices becoming smaller in size and better in performance, excellent discharge performance under high load is required for thermal batteries, i.e., high voltages have to be maintained at a large current discharge of 1 to 2 A/cm2 or more. Additionally, a decrease in the height of the stack, i.e. the height of the thermal battery is required, by increasing the voltage of the unit cell and reducing the number of the unit cell to be used.
When a thermal battery is discharged at a current density of about 0.5 A/cm2, the voltage of a unit cell in which the above composite materials including iron disulfide are used for the cathode active material is about 2.1 V, i.e., larger than the voltage of a unit cell in which iron disulfide alone is used for the cathode active material (approximately 1.8 V). Therefore, the height of a thermal battery can be decreased.
However, when a thermal battery is discharged at a current density of 1 to 2 A/cm2, i.e., when a large current discharge is carried out, the voltages of the above unit cell in which the composite materials including iron disulfide are used are about 1.7 to 1.9 V, about 1.6 to 1.9 V, and about 1.6 to 1.8 V, respectively, while the voltage of the unit cell in which iron disulfide alone is used for the cathode active material is about 1.6 to 1.8 V. Thus, when the large current discharge is carried out, there is no significant difference in the voltage between the unit cell in which the above composite materials including iron disulfide are used for the cathode active material and the unit cell in which iron disulfide alone is used for the cathode active material. That is, the effect of a high discharge voltage will be insignificant.
Therefore, to solve the above conventional problems, the present invention aims to provide a thermal battery with excellent discharge performance under high load, by increasing the voltage at a large current discharge.